The conventional borehale measurement method is restricted in measurement range or distance to a periphery of the bore hole when executing the method electrically or magnetically, thereby disabling a measurement between bore holes. A technique of solving this problem includes a seismic exploration method using an elastic wave or a shear wave. However, in the existing seismic exploration method, a steel casing for protecting the hole wall has a problem related to the coupling, and a vibration source has some problems, thereby disabling the satisfactory investment to be carried out. For example, there is disclosed a technique of emitting the vibration energy outside the hole wall by vibrating a mass up and down and left- and right-hand using hydraulic system, and closely contacting it to the hole wall by a clamp (U.S. Pat. Nos. 5,229,554, 4,923,030, 4,648,478, 4,991,685, 4,796,723, 5,031,717, 5,212,354, 5,113,966, and 4,805,725).
But, the vibration caused by a hydraulic system cannot exhibit the stable output in a wide frequency range, particularly a high frequency range of more than 1 kHz, which provides the defect that it is difficult to use a signal compression technique using a pseudo random signal, and the like. Also, the large output vibration source disclosed by the representative U.S. Pat. No. 4,805,725 is capable of transmitting the receivable energy by .enlarging the output without using the signal compression technique. However, the measurement is difficult in the high frequency range, thereby making it difficult to improve the measurement accuracy, and further it is impossible to carry out the measurement while changing the frequency, thereby making it difficult to calculate the information, such as the permeability values, which is obtained from the sound propagation characteristics. Moreover, a device installed on the ground for generating the large output becomes very large, thereby increasing the time and the cost which are required for the preparation of the measurement such as the movement of the measuring equipment, and the measurement itself.
On the other hand, The vibration source (U.S. Pat. No. 5,042,611), called bender type, in which the vibrator vibrates like a cord is capable of emitting the pseudo random signal and keeping high output. However, it has a problem on the durability of the vibrator, which makes it impractical.
According to the prior invention which was previously invented by the inventor, the permeability and the porosity of the sediments, and the nature of the pore-fluid (oil, gas, and/or water) within the sediment pore space is remotely determined and imaged through the crosswell tomography (See Japanese Patent Provisional Publication (Kokai) No. 4-198794), and the like, which was proposed by the present inventor. The crosswell tomography enables to measure the sound velocity and attenuation at high frequencies (typically 500 to 6000 Hz).
However, this acoustic tomography has been limited to a relatively low frequencies of the order of 100 Hz in order to sufficiently ensure the measurement distance. Acquisition of high frequency (500-6000 Hz) seismograms across two wells separated a long distance has been difficult because of the following reasons:
1. High attenuation of sound through sediments, PA1 2. High level of ambient noise in the receiver wells, PA1 3. Noise through receiver cable caused by wind and ground machines, and PA1 4. The loss of the source energy by well casings.
This problem has been overcome substantially by the technique of the pseudo random binary sequence code measurement (hereinafter referred to as "PRBS measurement") by the present inventor, which is one of the signal compression techniques. This technique utilizes an omnidirectional acoustic source 14 which continuously generates a PRBS signal into all directions in a well 12 drilled in a formation 10, for transmitting underground acoustic wave, a vibration receiver comprising an array of hydrophones in another well, and a real time PRBS recorder capable of averaging and cross-correlating in real time without being subjected to the restriction of the measuring time and the length. The boring investigation of the well reveals that the obtained two-dimensional image of porosity, permeability, shear strength, and the like are correct.
For the case of PVC (vinyl chloride) cased wells having a casing 16 comprising a PVC pipe, such as for ground water and well for foundation engineering, crosswell tomography measurements have been successfully made to crosswell distance of up to 600 m with PRBS frequencies up to 6000 Hz using the above technique. From this data, accurate two-dimensional images of the permeability, porosity and shear strength have been obtained. For the PVC pipe, the acoustic impedance of well fluid (water) is approximately the same as that of PVC, so that the acoustic source 14 does not have the energy loss when the energy passes through the substances which are different in impedance from one another, which enables sufficient energy to effectively propagate in the ground through the casing 16.
However, for the steel cased wells, such as a deep well for producing oil, the impedance of steel is two order of magnitude larger than that of water or oil, thereby increasing the transmission loss at the casing pipe wall as shown by the arrow A, and then causing the energy to be dispersed upward and downward, as shown by the arrow B, which disables the acoustic source 14 to transmit only very low acoustic energy to the formation 10 through this steel casing 16 for oil well. For example, in the experiment of the crosswell tomography by the original PRBS system using the conventional acoustic source, which was carried out at the production oil field of Trinidad Tobago in October 1996, new oil reservoir was discovered by the successful crosswell tomography at the PRBS frequency of 500 Hz in 300 m deep.times.100 m wide section. Also, it was discovered that the conventional method has the loss of the acoustic energy due to the steel casing, which was as large as substantially 55 dB, that is, 99%. In other words, the steel casing can pass only 1% of the acoustic energy to the formation.
On the other hand, as the source for providing a vibration in the ground, it is expected to dispose a clamping mechanism 24 on a source vibrator 20 provided with an axial direction-wise actuator 22, and then to directly fix the both sides of the upper portion of the axial direction-wise actuator 22 to the casing 16 as shown in FIG. 19. In the drawing, reference numeral 26 designates a motor pump, and 28 a cable.
However, thus clamping the source vibrator 20 at its both sides causes the vibration generated by the axial direction-wise actuator 22 to be unfavorably released in a direction opposite to such a direction that the vibration should be propagated, which deteriorates the efficiency. Further, the axial direction-wise actuator 22 vibrates in the axial direction, which enables only the up and down transverse wave (shear wave) to propagate in the formation 10, but which disables the longitudinal wave (compression wave) to propagate therein. Further, the frequency characteristics are very bad, which provides problems that although the propagation at near 100 Hz is ensured, the propagation at the other frequency ranges are not ensured, and the like.